1. Field of the Invention
This invention relates to an interlayer film for laminated glass which has a superior sound insulation quality.
2. The Prior Art
In general, laminated glass comprising two sheets of glass sandwiching a resin film is superior in terms of safety because fragments do not fly about even if the glass is broken, and therefore, for example, it is widely used as windshield glass for transportation vehicles such as automobiles and window glass for buildings.
Of these interlayer films for laminated film, the polyvinyl butyral resin film plasticized by the addition of a plasticizer has superior adhesiveness to glass, high tensile strength, and high transparency, and therefore the laminated glass using this film is particularly suitable as windshield glass for vehicles.
In general, sound insulation performance is indicated as the transmission loss at varying frequencies. Sound insulation is prescribed in JIS A4708 as a constant value at 500 Hz or above, depending on the sound insulation grade, as shown in FIG. i with a solid line. The sound insulation of glass plates substantially decreases in the frequency range centered around 2000 Hz because of the coincidence effect (the dotted-line valley shown in FIG. 1 corresponds to the reduction of the sound insulation performance caused by the coincidence effect, where the prescribed sound insulation performance is not maintained)- The "coincidence effect" mentioned here stands for the phenomenon wherein, when the sound wave hits the glass plate, the rigidity and the inertia of the glass plate cause propagation of transverse waves on the glass surface, and these transverse waves resonate with the incident sound, resulting in sound transmission.
Although conventional laminated glass is superior in terms of the prevention of scattered fragments, it cannot avoid the reduction in sound insulation performance caused by the coincidence effect in the frequency range centered around 2000 Hz, and thus improvements are called for in this respect.
On the other hand, based on the loudness-level contour, it is known that human hearing is much more sensitive to sound in the range of 1000-6000 Hz compared with other frequency ranges, indicating that it is very important for purposes of sound control to eliminate the drop in sound insulation performance caused by the coincidence effect.
In order to improve the sound insulation performance of laminated glass, it is necessary to mitigate the coincidence effect described above to prevent the decrease in the minimum transmission loss caused by the coincidence effect (hereafter, this minimum is referred to as the "TL value", as shown in FIG. 1).
There have been various countermeasures proposed as means to prevent the decrease in the TL value, such as an increase in the mass of the laminated glass, multi-layered glass, segmentation of the glass area and improvement of the means to support the glass plate. However, none of these have produced sufficiently satisfactory effects, and their cost is not sufficiently low for practical use.
Recently, the requirement for sound insulation performance has increased more and more, and, for example, superior sound insulation performance around room temperature is required for building window glass. That is, what is required as superior sound insulation performance is that the temperature at which the sound insulation performance is the highest, determined by plotting the transmission loss (TL value) against the temperature (temperature of the maximum sound insulation performance=TLmax temperature), is around room temperature, and that the maximum value of the sound insulation performance (maximum value of the sound insulation performance=TLmax value) itself is high.
However, conventional laminated glass for automobile windshield glass using plasticized polyvinyl butyral resin films has a problem in that the temperature of the maximum sound insulation performance is higher than room temperature and that the sound insulation performance is not good around room temperature.
As for prior arts which attempted to improve the sound insulation performance of laminated glass, for example, Tokko (Japanese examined patent publication) Sho-46-5830 describes an interlayer film comprising a resin, such as polyvinyl butyral, which has an approximately 3 times higher fluidity than the fluidity of normal interlayer films in the flow stage in the molding. However, this has a serious problem in that the absolute value of the sound insulation performance of the laminated glass is low.
Tokkai (Japanese unexamined patent publication) Hei-2-229742 describes an interlayer film comprising laminated layers of polymer films with a glass transition temperature of 15.degree. C. or lower, such as a vinyl chloride-ethylene-glycidyl methacrylate co-polymer film, and a plasticized polyvinyl acetal film, for example. However, this does not show a sound insulation performance higher than grade Ts-35 in the sound insulation grade system according to JIS A4706, and furthermore, the temperature range in which it insulates against sound is limited, thus it cannot have a good sound insulation performance over a wide temperature range.
Tokkai Sho-62-37148 proposes, for the purpose of preventing a decrease in the sound insulation performance in the coincidence range of the rigid plate (glass plate), an interlayer film comprising laminated layers of two or more viscoelastic materials with different acoustic resistances, such as a polymethyl methacrylate film and a vinyl chloride-ethylene-glycidyl methacrylate co-polymer film, for example. However, this does not have a satisfactory sound insulation performance in the initial stage, and furthermore, it has a durability problem in that the performance deteriorates over long term use. The cause of this is that these resins undergo mass transfers because of the diffusion action at the interface, resulting in a diminished effectiveness as a constraint layer at the interface.
Tokkai Sho-60-27630 proposes an interlayer film comprising a poly vinyl chloride resin containing a plasticizer as a means to increase the sound insulation in the initial stage. Although this interlayer film has somewhat better initial sound insulation, the lamination process conditions have to be changed from the conditions for polyvinyl butyral interlayer films. Currently, polyvinyl butyral interlayer films are still the mainstream as the materials for laminated glass interlayer films, and, in the present situation, the preliminary bonding conditions of the lamination process are set at the conditions for polyvinyl butyral interlayer films.
Tokkai Sho-62-278148 proposes an interlayer film for laminated glass with low self-adhesion comprising a polyvinyl acetal resin whose acetal groups have a carbon number of 6-10 and a plasticizer. However, this interlayer film does not quite have sufficient sound insulation.
Tokkai Sho-51-106190 proposes to obtain a structure which has a damping effect in a wide temperature range by laminating two or more resins with different glass transition temperatures. This structure is found to have an improved damping effect over a wide temperature range. However, this Tokkai does not have descriptions about sound insulation and transparency necessary for laminated glass, and this structure does not satisfy the requirements for safety glass, such as high shock energy absorbion, and the prevention of scattering of fragments when the glass breaks.
Tokkai Sho-62-10106 proposes polyvinyl butyral with moderate cross-links by means of inter-molecular bonding. However, the laminated glass which comprises this resin and an added plasticizer has a sound insulation of only about 30 dB around room temperature.
Tokkai Hei-3-124441 describes an interlayer film comprising two or more laminated layers of polyvinyl butyral containing a plasticizer and with different degrees of polymerizations. However, the desired sound insulation performance cannot possibly be obtained by just laminating two or more layers of polyvinyl butyral with different degrees of polymerization.
Tokkai Sho-62-9928 describes extruded sheets comprising polyvinyl butyral resin with moderate cross-links by means of stable inter-molecular bonding and a plasticizer. However, this does not even qualify as grade JIS 30 around room temperature, and therefore the sound insulation performance is too poor.
Tokkai Hei-4-254444 proposes an interlayer film comprising laminated layers of a film of polyvinyl acetal resin whose acetal groups have a carbon number of 6-10 and a plasticizer, and a film of polyvinyl acetal resin whose acetal groups have a carbon number of 1-4 and a plasticizer. This interlayer film is found to have improved sound insulation, and its sound insulation performance does not vary significantly with varying temperatures. However, these effects are still not sufficient.
Of the interlayer films disclosed in Tokkai Hei-4-254444 described above, the interlayer film comprising films of polyvinyl butyral and a plasticizer laminated on both sides of a film of polyvinyl acetal whose acetal groups have a carbon number of 6-8 and a plasticizer qualifies as grade JIS 35, showing an improvement in the sound insulation performance. However, this does not show good sound insulation over a wide temperature range, which is an important requirement for a sound insulating interlayer film.
Tokko-Sho 40-3267 discloses the interlayer for safety laminated glass having 2 polyvinyl butyral films. However, this patent does not discuss a sound insulation quality. As we have shown so far, laminated glass which shows a superior sound insulation performance over a particularly wide temperature range and for a long period of time cannot be configured using the interlayer films of the prior arts described above.
Considering the points described above, the object of this invention is to provide a sound insulating interlayer film for laminated glass which does not hamper the basic properties necessary for laminated glass, such as transparency, weather resistance, shock energy absorption and adhesion to glass, prevents a decrease in the TL value by mitigating the coincidence effect, and maintains a superior sound insulation performance over a wide temperature range for a long period of time.